x/mobile: Windows event handling back-pressure

[egonelbre]

Using go 1.7.1/Windows 7/amd64. x/mobile at 16fd47fa04e0a40c643b41876f91f3d06dded396 + fix from #16991.

This is issue can easily be seen when resizing the window: basic and centering.

theApp event queue contains an infinite buffer in x\mobile\app\app.go and when Windows sends messages at a high rate -- e.g. during a window resize -- the whole interface becomes laggy.

Windows sends events as fast as you let it -- but since there is no back-pressure in x\mobile\app\shiny.go the queue gets flooded with Paint and Resize events.

PS: It could become an issue in shiny as well, because it also contains async handling of messages. However ATM there is no infinite queue, at least I couldn't find one, hence it provides some back-pressure which prevents Windows from flooding the queue.

[egonelbre]

Note, this buffering can be avoided by detaching handling paint event and other events from doing the actual painting:

func main() {
	app.Main(func(a app.App) {
		var glctx gl.Context
		var sz size.Event 
		tick := time.NewTicker(15 * time.Millisecond)
		events := a.Events()
		for {
			select {
			case e := <-events:
				switch e := a.Filter(e).(type) {
					case lifecycle.Event:
						switch e.Crosses(lifecycle.StageVisible) {
						case lifecycle.CrossOn:
							glctx, _ = e.DrawContext.(gl.Context)
							onStart(glctx)
						case lifecycle.CrossOff:
							onStop(glctx)
							glctx = nil
						}
					case size.Event:
						sz = e
						touchX = float32(sz.WidthPx / 2)
						touchY = float32(sz.HeightPx / 2)
					case paint.Event:
						// nothing to do here
					case touch.Event:
						touchX = e.X
						touchY = e.Y
					} 
			case <-tick:
				if glctx != nil {
					onPaint(glctx, sz)
					a.Publish()
				}
			}
		}
	})
}

However, here the painting is not timed correctly together with resizing, which can cause some distortion and we cannot handle inside size.Event due to potential flooding. There's also a potential for not handling frames at the correct time.